doc: Improve style of table of supported feature header

Border was bogus -- fix CSS rule to provide a more subtle look.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

doc/_extensions/zephyr/domain/static/css/board.css

@@ -110,6 +110,7 @@
     th {
         background: var(--navbar-background-color);
         color: var(--navbar-level-1-color);
+        border: none !important;
         font-weight: 600;
         text-transform: uppercase;
         font-size: 0.8em;